1. Field of the Invention
The present invention concerns a radio-frequency transmission system for a magnetic resonance system, of the type having a radio-frequency amplifier and a signal splitter with two inputs and two outputs, wherein the power of a radio-frequency signal provided at any one of the two inputs is divided between the two outputs. A first input of the two inputs of the signal splitter is thereby coupled with the output of the radio-frequency amplifier, and the two outputs of the signal splitter respectively serve to connect to different inputs of a transmission antenna of the magnetic resonance system in order to feed the output signals present at the two outputs of the signal splitter into the transmission antenna in operation. Moreover, the invention concerns a magnetic resonance system with a transmission antenna that has at least two inputs to feed in radio-frequency signals, and with such a radio-frequency transmission system. Furthermore, the invention concerns a method to control a magnetic resonance system to acquire magnetic resonance image data of an examination subject, in which method, to generate a radio-frequency field in an examination volume, radio-frequency signals emitted with a specific signal power by a radio-frequency amplifier are directed to an input of a signal splitter in which the power of the radio-frequency signal is divided into two output signals present at two outputs of the signal splitter, these output signals being respectively fed into different inputs of a transmission antenna.
2. Description of the Prior Art
Magnetic resonance tomography is a widespread technique to acquire images of the inside of the body of a living examination subject. In order to acquire an image with this method, the body or a body part of the patient or test subject to be examined must initially be exposed to an optimally homogeneous, static basic magnetic field, which is generated by a basic magnetic field of the magnetic resonance system. During the acquisition of the magnetic resonance images, rapidly switched gradient fields (generated by gradient coils) are superimposed on this basic magnetic field for spatial coding. Moreover, radio-frequency pulses of a defined field strength (known as the “B1 field”) are radiated into the examination subject with radio-frequency antennas. By means of these radio-frequency pulses, the atoms in the examination subject are excited such that they are deflected from their equilibrium state parallel to the basic magnetic field by what is known as an “excitation flip angle”. The nuclear spins then precess around the direction of the basic magnetic field. The magnetic resonance signals thereby generated are acquired by radio-frequency acquisition antennas. Magnetic resonance images of the examination subject are generated on the basis of the acquired magnetic resonance signals.
To radiate the required radio-frequency pulses into the patient positioning region of the apparatus, the magnetic resonance system typically has an antenna structure permanently installed in the housing of a scanner. This radio-frequency tranmission antenna is also designated as a body coil. For example, the frequently used birdcage structure is formed by a number of conductor rods arranged around the patient space and running parallel to the basic field direction, these conductor rods being connected with one another via annular conductors at the front side of the coil. As an alternative, there are also other antenna structures permanently installed in the housing, for example saddle coils. Moreover, local coils can also be used that are arranged directly on the body of the patient. These local coils have conventionally been used only as acquisition coils. Classical magnetic resonance systems have essentially only one transmission channel to emit the B1 field, meaning that there exists only one transmission line that leads from the radio-frequency amplifier to the transmission antenna. Insofar as the antenna (for example a birdcage antenna) is fashioned such that a circular polarized field can be emitted, the radio-frequency signal coming from the radio-frequency amplifier can initially be divided into two signals via a signal splitter. A component known as a hybrid module is typically used for this purpose. The signal splitter is therefore also designated in the following as a “hybrid” for short. The output signals have a phase difference that is primarily predetermined by the signal splitter that is employed. Often a hybrid known as a π/2 hybrid is used in which the output signals are shifted opposite one another by 90° in terms of their phase. The two signals are then fed into the antenna structure via two signal lines to precisely defined connection points, or inputs.
Such a classical design is schematically presented in FIG. 1. Starting from a radio-frequency signal generator 11, the radio-frequency signals are amplified in the radio-frequency amplifier 21 so that they possess a sufficient transmission power. The distribution of the transmission power to two transmission channels at the two outputs 26, 27 of the hybrid 23 ensues in the hybrid 23. The two signal portions are relayed to two inputs 16, 17 of the transmission antenna 15 and fed in there. The employed hybrid 23 is normally a 4-gate hybrid with an additional input 25. This is typically terminated with a termination resistor 28, typically 50 Ω, which serves as a “sump” for transmission power reflected back from the antenna 15 or returning due to overcouplings between the antenna inputs 16, 17. The precise conductor and reflection behavior of such a design is discussed in more detail below.
In this design, the distribution of the B1 field is frozen or fixed via the distribution to the two transmission channels with the phases of 0° and 90° and cannot be adapted to the current conditions of the pending measurement.
Particularly in newer magnetic resonance systems with basic magnetic field strengths greater than three Tesla, considerable eddy currents are frequently induced in the patient upon radiation of the radio-frequency pulses. As a result, the actual homogeneous radiated B1 field is more or less strongly distorted in the examination volume. The influence of the patient body on the B1 field is thereby strongly dependent on the body stature of the patient and the proportions of the individual tissue types, among other things. For example, a very corpulent patient causes a circularly polarized magnetic field to be severely distorted into an elliptical field. This distortion is not as severe for thinner patients. In individual cases this can lead to the situation that a magnetic resonance measurement is unreliable in specific body regions of the patient and delivers unusable results.
In order to be able to suitably influence the structure of the radiated magnetic field in an optimally detailed manner in all regions of the examination volume, and in particular in order to achieve an optimally good homogeneity of the B1 field in an examination volume via a compensation of the possible distortions, local field corrections have conventionally been implemented by the use of dielectric cushions (pillows), for example.
Presently, individual adjustments of the amplitude and phase values of the radio-frequency pulses emitted from each transmission channel are being investigated as an additional approach to homogenization of the B1 field. The spatial distribution of the B1 field can be influenced thereby, with the goal of generating an optimally homogeneous radio-frequency field in the examination subject or in the examination volume while taking into account the field distortions to be expected. One development of this approach is the use of multiple, separately controllable antenna elements. An example of this is explained in DE 101 24 465 A 1, which describes an antenna with a number of separately controllable antenna elements. This means that every transmission channel has a separate antenna element. Alternatively, different feed lines connected to a common antenna structure can be supplied via individually controllable transmission channels.
A particularly simple variant of this is shown in FIG. 2. FIG. 2 shows a typical 2-port antenna as is also used in the design according to FIG. 1. As before, a radio-frequency signal generated by a radio-frequency signal generator 11 is likewise amplified in a radio-frequency power amplifier 21 and divided via a hybrid 23 between the two antenna inputs 16, 17. However, a difference from the design according to FIG. 1 is that the output of a second radio-frequency power amplifier 21′ is connected to the second input 25 of the 4-gate hybrid 23. A second, independent radio-frequency signal generator to supply the second radio-frequency power amplifier 21 can also likewise be used. Given this design, an arbitrary weighting of the signals fed into the transmission antenna 14 at the two inputs 16, 17 is possible in that a different amplification ensues via the two radio-frequency amplifiers 21, 21′. For example, as before the first amplifier 21 can deliver the majority of the transmission power. Only an additional portion in order to adjust the field distribution as desired, and in particular to improve the field homogeneity at least in the region of interest of the patient, then comes from the second amplifier 21′. However, a second radio-frequency amplifier is required for this design, which leads to significant additional costs.